The technical field of this invention concerns analytical devices for use in determining the presence of a bindable target substance (i.e. analyte) in a liquid sample suspected of containing such substance. Numerous analytical devices, in various configurations, have been used for this purpose. Many of these devices employ reaction membranes onto which a receptor capable of specifically binding to the target substance, is immobilized. In the assay that employs these types of devices, typically the sample to be tested is applied to the reaction membrane. If the target substance is present in the sample, it will bind to the immobilized receptor. Various methods are used to determine whether the target substance has bound to the receptor, thus indicating its presence in the sample. In one commonly used method, an antibody that is capable of specifically binding to the target substance and that is attached to a detectable label is applied to the membrane.
Membrane-based immunoassays and devices have greatly simplified medical diagnostics. In the past, enzyme-linked immunosorbent assays (ELISA) were the most common type of diagnostic assay. These assays were usually performed by skilled technicians in a clinical laboratory. With a typical ELISA, relatively long incubation times are required. Therefore, the assay usually takes over an hour to perform. Additionally, ELISAs are designed for testing samples in batches. Therefore, a sample is generally not tested until a sufficient number of samples have been obtained for testing. As a result, a patient may not receive test results until days after providing a sample. Membrane-based immunoassays have the advantage that they can be performed individually or in batch. Thus, a membrane-based immunoassays can be performed in a doctor's office rather than in a clinical laboratory. Individual tests can be performed, and results obtained, usually under 10 minutes. Quantitative results can also be provided by special instruments designed to read the test results.
The sensitivity of reaction-membrane type immunoassays (i.e. the ability to detect very low levels of target substance) can be increased if the sample is concentrated through the reaction membrane. With some devices, concentration of the sample through the reaction membrane is achieved by having an absorbent material beneath the reaction membrane that draws the sample, which is added to the top of the membrane, through the membrane and into the absorbent material below. Membrane-based immunoassays that utilize absorbent materials to concentrate sample are exemplified in U.S. Pat. No. 5,006,464 to Chu et al., U.S. Pat. No. 4,818,677 to Hay-Kaufman et al., and U.S. Pat. No. 4,632,901 to Valkirs et al., and U.S. Pat. No. 5,185,127 to Vonk et al.
The membrane and absorbent material of the analytical devices described in each of the patents referenced above are contained in a plastic housing having a top member and a bottom member which are joined together under compression to hold the membrane and absorbent material in place and in contact with each other. The use of sufficient compression facilitates the downward flow of the sample through the membrane. If there is insufficient compression, sample would tend to flow laterally across the membrane, thus reducing the amount of sample that flows through the center of the reaction membrane where the receptor is usually located. These types of analytical devices are typically assembled individually which can make the manufacturing process complicated and costly.
There is a continued need to further simplify membrane-based immunoassays and analytical devices therefor, as well as the analytical device manufacturing process to make them easier to use and more affordable. This is particularly true in developing countries, where a simple, rapid, sensitive, specific, storage-stable and economical diagnostic assay and analytical device would be ideal.